In the bowl of a food processor pulse together flours, baking powder, salt and sugar until incorporated. Add butter, and pulse until mixture resembles coarse crumbs. Gradually stream the the beaten egg into the flour until the dough just comes together. Turn out the dough onto a clean work surface, shape it into a flattened disk and wrap in plastic. Refrigerate for 1hour.
Preheat the oven to 180°C. Line two baking sheets with parchment paper.
On a well floured surface, roll dough until it’s 3-5 mm thick. Cut hearts or other shapes with cookie cutters. Carefully transfer to prepared baking sheets. Re-roll scraps and cut out shapes. Repeat with remaining dough.
Bake the cookies for 8 minutes, rotating halfway through. Remove from oven as soon as they start to get color on the edges. They should stay light in color and only get a slight light-brown color on the bottom. While hot, the cookies are still a bit soft but once they cool down, they firm up. Cool completely on wire rack before filling. Plain cookies can be kept in an airtight container for up to 2 weeks.

To assemble the cookies, spread about a teaspoon of your desired filling on the bottom side of one cookie and top with another cookie. Sandwich the cookies together and transfer to an airtight container. The sandwiched cookies can be kept for 4-5 days, depending on your choice of fillings.